This invention relates to electroluminescent devices containing polymers, and to processes for the use of these devices.
In recent years, a great deal of research has been conducted into electroluminescent materials, that is to say materials which emit electromagnetic radiation (typically visible light) when an electric current flows through the polymer. Electroluminescent materials are potentially useful for the construction of image display devices, which could be very thin and lightweight, and could thus advantageously replace cathode ray tubes, gas plasma displays, liquid crystal displays and other types of image display devices.
Several different types of electroluminescent materials are known; see, generally as to development of such materials, International Patent Application No. PCT/GB90/00584 (Publication No. WO 90/13148). The first type to be developed was inorganic semiconductor materials such as gallium phosphide and zinc sulfide. However, such inorganic electroluminescent materials are not readily usable in large image display devices, and many of them suffer from practical drawbacks, including poor reliability. Accordingly, most recent research has concentrated on organic electroluminescent materials.
Many organic compounds, especially polycyclic arenes such as anthracene, perylene, pyrene and coronene, are electroluminescent. However, electroluminescent devices using these monomeric organic compounds suffer from poor reliability, and these organic compounds present difficulties in preparing the thin layers of the materials needed for use in practical electroluminescent image display devices, and the electrodes needed for electrical contact with such layers. Techniques such as sublimation of the organic material produce layers which are soft, prone to recrystallization and unable to support high temperature deposition of electrode layers, while techniques such as Langmuir-Blodgett film deposition produce films of poor quality, dilution of the active material and high cost of fabrication. Prior art electroluminescent devices formed from these materials, such as that described in U.S. Pat. No. 3,621,321, typically suffer from high power consumption and low light output.
Attempts have also been made to use solid solutions of monomeric organic electroluminescent materials in non-electroluminescent polymers as the active layer in electroluminescent devices; see, for example, U.S. Pat. No. 4,356,429. However, use of such solid solutions carries a substantial risk of phase separation by crystallization of the monomer out of the polymer, especially in environments where the electroluminescent device may be subjected to large changes in temperature. In addition, often it is difficult to find a non-electroluminescent polymer which can both conduct electricity and dissolve a large proportion of the active electroluminescent material to form the necessary solid solution. Finally, the use of a solid solution necessarily involves substantial dilution of the active electroluminescent material and thus lowers the maximum light flux from a given area of the electroluminescent device.
Accordingly, research has been carried out on electroluminescent polymers having an electroluminescent group incorporated into the polymer itself, particularly polymers containing a chain of electroluminescent groups (such as para-phenylene vinylene units) conjugated with one another along the chain; see, for example, International Patent Applications PCT/GB90/00584, PCT/GB91/01420 and PCT/GB91/01421 (Publication Nos. WO 90/13148, WO 92/03490 and WO 92/03491 respectively). These polymers do not suffer from the phase separation, solubility and stability problems encountered with the aforementioned solid solutions, and permit some control over the wavelength of the light emitted by variations in the chemistry of the electroluminescent polymer. For example, the aforementioned International Application PCT/GB91/01420 describes electroluminescent polymers comprising at least two different monomer units having different semiconductor bandgaps to control the conjugation length within the polymer, and hence the quantum efficiency and wavelength of light emitted, while the aforementioned International Application PCT/GB91/01421 describes electroluminescent polymers having first and second regions differing in optical properties. However, the copolymers described in these International Applications are random copolymers, in which conjugation extends through a plurality of repeating units of the polymer until interrupted by certain non-conjugated units, which are randomly distributed throughout the polymer chain. In such copolymers, considerable variation in conjugation length is possible because of the varying distances between the non-conjugated units, and because, where a long series of conjugated units occur together in the chain, conjugation may be interrupted by twisting of the chain, and such twisting occurs at essentially random intervals as the chains twist around one another in the polymeric matrix. The variation in conjugation length within these copolymers results in electroluminescent emission over a considerable range of wavelengths.
For certain applications of electroluminescent materials, such as flat panel displays, it is desirable to emit light within a narrow range of wavelengths. In addition, the electroluminescent polymer should not require the presence of additives which pose stability or phase separation problems. Finally, it is desirable that the electroluminescent polymer have good electroluminescent efficiency (which is measured as quanta emitted per electron injected); the electroluminescent efficiencies of prior art polymers are typically of the order of 0.1%, so that large amounts of electrical energy are required to produce an electroluminescent image display device which can be viewed in normal room lighting, as is essential if, for example, the image display device is to be used as a computer monitor or television screen.
It has now been found that electroluminescent polymers having desirable properties can be formed by providing a plurality of electroluminescent groupings in the polymer chain, adjacent pairs of these electroluminescent groupings being connected via a "substituted diphenyl linkage" (as explained in more detail below), this linkage being such that the adjacent electroluminescent groupings are essentially not conjugated with each other.